1. Field of the Invention
This invention relates to an ink feeding rate control method and an ink feeding rate control apparatus for adjusting ink feeding rates of a printing machine when performing a printing operation by using printing plates with images printed thereon based on printing data.
2. Description of the Related Art
In order to perform proper printing with a printing machine, it is necessary to control ink feeding rates properly. For controlling the ink feeding rates, it has been conventional practice to measure densities of control strips with a densitometer and determine from density data whether the ink feeding rates are proper or not. However, the density data from the control strips alone is not necessarily sufficient for attaining a proper color tone and the like for a picture area.
For this reason, a print quality measuring apparatus is used which provides control data for controlling the ink feeding rates of a printing machine. The control data is produced by comparing an image on reference paper and an image on an actual print. The reference paper is also called proof paper, and serves as a reference indicating a color tone of finished prints to obtain proper prints. Printing paper actually printed is also called sampling paper which is extracted by the operator from a discharge station of a printing machine at certain intervals during a printing operation. The printing is considered proper when the color tone on the sampling paper substantially coincides with the color tone on the reference paper.
However, for controlling the ink feeding rates of the printing machine by using reference paper in this way, the reference paper must be made ready for use in advance. To avoid this inconvenience, a printing apparatus disclosed in Japanese Unexamined Patent Publication No. 2001-235054 uses, instead of reference paper, image data (digital data) used in making printing plates. That is, the printing apparatus disclosed in Publication No. 2001-235054 controls ink feeding rates of a printing machine by comparing image data used in recording an image on printing plates and data obtained by photographing an image actually printed on printing paper.
Where, as described in Publication No. 2001-235054, image data used in recording an image on printing plates is used in place of reference paper, a problem arises that the image is not printed with a desired color tone because of printing characteristics variable from one printing machine to another. That is, image data may be created by taking color characteristics of a particular printing machine into account. When ink feeding rates are controlled by comparing this image data and data obtained by photographing printing paper printed by a different printing machine, the result is a problem that the ink feeding rates are controlled to target colors different from what is originally intended.